1. Field of the Invention
The present invention is directed to apparatus for testing planar reflective surfaces to determine the effective reflection capabilities thereof. More particularly, the present invention is directed to an apparatus for testing glass sheet used in very sensitive reflective surfaces to determine the flatness of the sheet and thus the reflective capabilities of the glass after silvering.
2. Description of the Prior Art
The use of optical signals to test numerous types of surfaces for roughness, or to otherwise determine surface characteristics, is a rapidly advancing art. The development of lasers, beam splitters, and dichroic mirrors makes it possible to utilize the reflective characteristics of the surface in order to determine the surface's own characteristics. If light is projected toward a flat reflective surface, normal thereto, the surface will reflect the light directly back to the source. However, any variations in the surface causing in incident angle other than 90.degree. results in the light being reflected away from the source. Using these basic principles, numerous systems have been suggested for determining surface characteristics.
U.S. Pat. No. 3,871,771 of Scott discloses a measuring system which utilizes polarized light from a laser and a beam splitter. The system measures roughness to a certain extent, but does not measure root mean square (RMS) variations in the suface characteristics. Jakeman et al in patent 3,971,956 measure the root mean square roughness of a surface by utilizing a laser. However, Jakeman et al direct the laser towards the surface, normal thereto, and then utilize two separated detectors at angles other than perpendicular to the suface to determine the RMS variations in the surface.
Ash in U.S. Pat. No. 3,836,787 suggest another system for measuring roughness. The reference utilizes an aperture that is smaller than the wavelength of the light which is generated at an angle normal to the surface of the object and oscillates the object behind the aperture to produce a modulated output signal. Variations in the modulated signal are then used to measure surface roughness.
U.S. Pat. No. 3,815,998 of Tietze utilizes collimated light directed normal to the surface being tested and captures the scattered reflected light with a lens. A knife edge at the focal point of the lens removes 50% of the scattered angles and the remaining light is allowed to be incident to a screen. The result is a contrast image formed on the screen. The system is very sensitive to slopes lying parallel to the knife edge, but is almost totally insensitive to slopes perpendicular to the knife edge.
U.S. Pat. No. 3,782,827 of Nisenson et al utilizes an analog system for collecting its data. A photometer is used behind an aperture for measuring the intensity of the reflected and scattered light. Nisenson rotates the sample to record the intensity as a function of rotation angle. Thus the system requires considerable work in order to measure the scatter characteristics and the physical structure results in a very small bandwidth.
In Smith U.S. Pat. No. 3,715,165 the specimen surface is investigated using a pencil of light which is brought to a point focus and oscillated longitudinally along the surface. Thus the reflected light varies in intensity at a downstream pinhole due to variations in the distance between the focus point and the surface. This intensity variation is used to produce a voltage which is recorded to determine the surface profile.
Rosenfeld et al in U.S. Pat. No. 3,885,875 generate a laser signal, pass the signal through a lens and reflect it off of a rotating surface. The reflected signal passes back through the lens and to the test article where it is reflected through the lens again, reversing its characteristics. Next, the signal is finally reflected by a beam splitter to a photosensitive element. The system does not use collimated light and it measures the characteristics against an artificial surface or profile. Rosenfeld et al further disclose various other known systems, and discuss the problems inherent in such systems.
Lastly, numerous other systems have been suggested in the prior art. Equipment has been designed to determine the surface characteristics of the test item in many ways, for example, U.S. Pat. No. 4,115,005 to Murata uses two converging light sources; U.S. Pat. No. 4,017,188 to Sawatari utilizes splitting of light and slits; and U.S. Pat. No. 3,998,553 of Hunter et al utilizes a spherical wavefront. Baker et al in U.S. Pat. No. 3,892,494 detects scratches on a surface by variations in intensity of the reflected focused light beam. Also, the art discloses the testing of different materials, for example, see U.S. Pat. No. 3,602,596 of Asthelmer et al (testing bearing races) and U.S. Pat. No. 3,734,626 of Roberts et al (testing magnetic discs).